The Instrument Landing System (ILS) is a standard type of aircraft landing system currently installed at many airports around the world. The ILS includes an antenna system arranged to provide glideslope signals radiated from a point near an aircraft runway toward an aircraft approach direction at an angle representing a desired glideslope angle for aircraft approaching the runway for landings. The glideslope angle, relative to a horizontal reference, is typically 3 degrees and the radiated glideslope signals typically include vertical guidance information which is detected by equipment aboard the aircraft to provide an indication to the pilot when the aircraft departs from the desired glide path. An aircraft can depart from the desired glide path by being too high (e.g., above the inclined center line of the radiated beam pattern) or too low (e.g., below such center line).
Existing types of ILS glideslope antennas provide a radiated beam pattern which is the composite of two portions of a radiated antenna pattern. These two portions are a direct radiated portion and a reflected portion. The actual radiated beam pattern is the resultant of adding the reflected portion to the direct radiated portion. Thus, in order to provide a 3 degree glideslope beam pattern, such existing ILS antennas radiate an antenna pattern which has closely equivalent radiated power at both +3 degrees and -3 degrees. Then, with reliance upon uniform reflection from the surface of the earth, the energy radiated at -3 degrees is reflected and additively combined with the energy radiated at +3 degrees. This prior form of ILS antenna utilizes horizontal polarization which provides an earth image that is very close to unity in relative amplitude with a 180 degree phase reversal, under ideal conditions. The ILS system depends upon the combination of the respective direct and reflected signals to provide the desired vertical guidance information. Consequently, the image (reflected) radiation is fundamental to the integrity of the guidance signal, which necessitates that provision must be made in order to monitor both the direct and the reflected signals on a continuing basis to ensure that a reliable guidance signal is provided.
Three basic problems are relevant to existing types of "imaging" glideslope antennas (i.e., antennas directly dependent upon the reliable presence of both direct and reflected signals to provide guidance signals). First, in installation of the antenna a substantially flat, open ground area must exist or be provided forward of the antenna in order to provide adequate, effective and uniform signal reflection. Second, under certain snow conditions (which are normally relatively rare) the image signal strength may be substantially reduced, so that its amplitude is much less than that of the direct signal. This situation could degrade or compromise the integrity of the guidance signal. Third, the image component of the guidance signal is difficult to monitor to insure continuing system integrity of operation. Monitors, for receiving image signal samples, which are located close to the glideslope antenna are not accurate. For accuracy, monitors must be located far from the glide path and are difficult to implement.
Objects of the present invention are, therefore, to provide new and improved glideslope antenna systems which avoid one or more of the disadvantages of prior antenna systems.
Additional objects are to provide new forms of non-imaging antennas which are particularly suitable for use in providing glideslope signals for aircraft landing operations.